Fiber cement is a well-known material employed in many building materials, such as siding, roofing and interior components, as well as pipes, particularly for waste water transport. Fiber cement typically comprises a mixture of cement (i.e., lime, silica and alumina), clay, a thickener, inorganic fillers such as calcium carbonate, and one or more fibrous materials. In the past, asbestos was commonly included as the fibrous material (see U.S. Pat. No. 4,216,043 to Gazzard et al.); because of the well-documented problems asbestos presents, now fiber cement typically includes a natural or synthetic fiber, such as acrylic, aramid, polyvinyl alcohol, polypropylene, cellulose or cotton. Fiber cement is popular for the aforementioned applications because of its combination of strength, rigidity, impact resistance, hydrolytic stability, and low thermal expansion/contraction coefficient.
To be used in siding or roofing components, fiber cement is often formed in sheets or tubes that can be used "as is" or later cut or otherwise fashioned into a desired shape. One technique of forming fiber cement articles (known as the Hatschek process) involves creating an aqueous fiber cement slurry of the components described above, depositing the slurry as a thin sheet or web on a porous fabric belt, and conveying the slurry over and through a series of rollers to flatten and shape the slurry. As the slurry is conveyed, moisture contained therein drains through openings in the fabric. Moisture removal is typically augmented by the application of vacuum to the slurry through the fabric (usually via a suction box located beneath the porous fabric). After passing through a set of press rolls, the fiber cement web can be dried and cut into individual sheets, collected on a collection cylinder for subsequent unrolling and cutting into individual sheets, or collected as a series of overlying layers on a collecting cylinder that ultimately forms a fiber cement tube.
The porous fabric used to support the slurry as moisture is removed is typically woven from very coarse (between about 2500 and 3000 dtex) polyamide yarns. Most commonly, the yarns are woven in a "plain weave" pattern, although other patterns, such as twills and satins, have also been used. Once they are woven, the yarns are covered on the "sheet side" of the fabric (i.e., the side of the fabric that contacts the fiber cement slurry) with a batt layer; on some occasions, the "machine side" of the fabric (i.e., the side of the fabric that does not contact the slurry directly) is also covered with a batt layer. The batt layer assists in the "pick-up" of the slurry from a vat or other container for processing. Because of the presence of the batt layer(s), the fabric is typically referred to as a fiber cement "felt."
Coarse yarns have been employed in fiber cement felts because of the severe conditions the felt experiences during processing. For example, fiber cement felts are typically exposed to high load conditions by the forming machine. Also, there can be significant variations in tension over the felt length on the fiber cement machine, as tension may vary from as low as 2 kilopounds/cm after the forming roll to as high as 15 kilopounds/cm over suction boxes. As a result, coarse yarns having high "tenacity" and resilience have been employed. However, because the yarns are coarse, such felts have a tendency to mark the surface of the fiber cement product formed thereon, sometimes to a sufficient degree that smoothing of the surface in a subsequent operation may be required. Further, fiber cement felts are prone to "blinding" (the filling of the openings in the fabric mesh with fiber cement slurry) and typically must be cleaned frequently and may be removed (depending on machine conditions such as speed and load) after as little as one week. Also, such felts tend to suffer significant "compaction" (the tendency of the felt to decrease in thickness) with use. Compaction is detrimental to operation in that, as the felt decreases in thickness, the pressure exerted on the fiber cement by the pressing rolls can decrease, thereby altering the surface characteristics as well as overall physical properties of the sheet. Also, some compaction may be localized, with the result that the fiber cement can have areas of different thickness. Accordingly, compacted felts are typically replaced.